"Senior leader AB Bardhan, a veteran of Left movement was admitted to GB Pant Hospital after he complained of breathlessness, is still critical but fighting. He had brain hemorrhage," CPI said in a statement.
CPI said its party headquarters has received "numerous" calls from its state unit leaders and other political parties from India and foreign fraternal outfits as the news about his hospitalisation spread.
NCP chief Sharad Pawar, ex-CPI (M) general secretary Prakash Karat and party politburo member Brinda Karat were amongst those who visited the hospital, besides CPI rank and file, to enquire about the senior leader's health.
Earlier in the day, party national secretary D Raja had said the condition of Bardhan was "very critical".
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He later rose to become the General Secretary and then the President of the All India Trade Union Congress, the oldest trade union in India.
Bardhan moved to Delhi politics in the 1990s and became the Deputy General Secretary of the CPI. He succeeded Indrajit Gupta as the General Secretary of the party in 1996.